Naturally occurring element X exists in three isotopic forms: X-28 (27.977 amu, 92.21% abundance), X-29 (28.976 amu, 4.70% abundance), and X-30 (29.974 amu, 3.09% abundance).

Question

Naturally occurring element X exists in three isotopic forms: X-28 (27.977 amu, 92.21% abundance), X-29 (28.976 amu, 4.70% abundance), and X-30 (29.974 amu, 3.09% abundance). Calculate the atomic weight of X. 

48.7 amu

28.5 amu

29.0 amu

28.1 amu

Answer 1

The atomic weight of the element will be a weighted average of the isotopes based on the relative abundance:

(27.977 x 0.9221) + (28.976 x 0.0470) + (29.974 x 0.0309) = 25.798 + 1.362 + 0.926 = 28.08 = 28.1 amu